Depends on which Piefed instance you're trying to contact and for what reason.
In the cases of feature requests a good place to start would be piefed.social/c/piefed_meta - !piefed_meta@piefed.social

If it's about contacting a specific piefed instance then check their about page, f.e.: https://piefed.europe.pub/about

If it's about helping out with developing etcetera then this page has links that will bring you to the right place: https://join.piefed.social/